Electric heating element



Sept. 9, 1952 p. B. cox 2,610,286

ELECTRIC HEATING ELEMENT Filed April 22, 1949 ,A INVENTOR.

17u/wan bx BI (mi.

HT rara/VE YS Patented Sept. 9, 1952 UNITEDSTATES `PATENT OFFICE ELECTRIC HEATING ELEMENT Duncan B. Cox, Woodbury, N. Y. l Application April 22, 1949, serial No. s8,949

'device oi?v the above character lwhich willlbe self- ,fusing when it becomes overheated and, there- "fore, is particularly safe for use as a heating element and which at the same time is so constructed and assembled that it is adequately strong andcan withstand the tensile stresses to which devices of. this type are subjected.

, A further object of the invention is to provide al device of the above character in which a tension or stress member is associated with the conv ductor or resistance wire in such a manner that the tensile stresses are sustained directly by the member rather than by the wire.

Another object of the invention is to provide improved heating devices incorporating conductors or resistance elements of the above type in `which the assembly and connection is so arranged that tlietensile stresses are necessarily transmitted through the tension member.

` In the accompanying drawing:

Fig. 1 isv a view of an improved conductor or resistance element embodying my invention;

Fig. 2 is a similar view of the conductor or resistance element having an outer insulating and protective casingr applied thereto;

Fig. 3 is a partially sectional view with the central portion omittedcf'a heating tape having my improved resistance element incorporated there- Fig. `4 vis y partially sectional view` at right angles to Fig. 3; i l y Y Fig. 5.is a cross sectional view of the heating tape shown in'Figs. 3 and 4; v

Fig. 6 is a diagrammatic plan view of an electric: heating -blanket having my improved resistance element incorporated therein; and

Fig. 7 is a detailed view of the connection between the resistance element and lead wires.

My improved conductor and resistance element is shown most 'clearly at I0 in Figs. l and 2 and comprises'generally an'elongated attenuated conductorl I 'made of a low fusing metal surrounded vby a stress carrying insulating casing I2, the lat- 3 Claims. (Cl. 219-46) one or more of the following-copper, antimony, tin, bismuth, arsenic or silver. Tin, cadmium,

` bismuth by themselves or as the major constituents of an alloy can also be used in place cf lead or zinc; but these are not economically attractive at the present time.

The diameter of the conductor may vary in accordance with'the desired resistance for the metal being used. I have found, however, that the practical lower limit is .005" and the practical upper limit is .036 and the diameter should be conned within these limits. Where the conductor or resistance is to be used in a heating unit of the type shown in Figs. 3, 4 and 5 and hereinafter more fully described, I have found that satisfactory results are obtained by making the conductor out of 4% antimonial lead with a diameter of .0119. This alloy melts at about 572 F., but in the form of extremely fine wire and when covered by a suitable layer of glass ber insulation it provides a. high degree of safety against fire hazards, in that it will readily fuse and open the circuit if inadvertently connected to too high a potential or if a short circuit should develop due to any mechanical damage to the insulation. I have also found that if a heating cable, blanket or pad using this new resistance element is allowed to operate under conditions which restrict the heat dissipation, excessive temperatures, which might cause a fire if any of the commonly known resistance wires were used, are prevented by the fusing of the conductor.

Resistance wire of the types and in the sizes used in my invention are extremely flexible and can withstand without damage a very large number of repeated sharp bends. However, the stress which such Wire can withstand without stretching is extremely low, both because the metal is soft and weak and because of the relatively ne diameter. For example, the .0119 4% antimonial lead wire will break with an applied load of less than vone-half pound. Although the wire can stretch without breaking, any stretching reduces the diameterof the wire and causes an increase'in its resistance which is approximately proportional to the amount of stretch. If more than a nominal increase in resistance occurs as the result of local stretching, the area affected will release more heat in operation than adjacent areas. This causes a local hot spot, which, in turn, will further increase the local heat dissipation because of the relatively large temperature coeiiicient of resistance of the wire. Therefore, wire which has been subject to local stretching is not suitable for use as a heating element. I

trated vby lthe following experiment. :monialleadwire -.020 in diameter and Y10" .long wasglaid onza smooth'insulating sheet 'With-all have found that it is possible to eliminate any local stretching by applying a tension member or stress-carrying insulating casing, preferably in the form of a special tubular braid l2 of glass ber, to the wire as the iirst operation after it is drawn, When this braid is being applied, the wire is subjected to only enough tension to cause it to unroll from its spool, a force of one or two ounces, but the braid is withdrawn from the braiding machine under high tension,y so that virtually all of the stretch in the fibers which exists as the result of the braided construction is removed before the braid comes in contact with the wire. A special impregnation' treatmentis then used to prevent the braid from loosening up and contracting in length whenthe tension is removed. Among the materials whichl have lfound suitable for this impregnation are compounds of high melting waxes and wateremulsions of plasticized polyvinyl chloride. The ten- -sion on the- ,braid.must-.be' maintained lr.luring\ and .-after'-,the application of i the -impregnating compound-.until it has seteither byI cooling,A asin, the

caserof `thehotmelt, .or by evaporation `,of =the watery heatcuringand. cooling,as-.in.the caseof the emulsion. Thereafter, .no specialcare Vis needed inhandling the resistance.elementwith itsimpregnated braided. covering, lsince ,the-braid .provides adequate protectionforgthe wire.

:As ,stated above 4the tubular .stressfcarrying casingispreferablymadein the formoi albraid and from-.glass fibers. .'Glass 'iberis-l particularly k:suited for this Aapplication 'due 4.to lits high strength about;25(l,00,0.k p. s. i.) and the fact.. that This higher speed reduces the :angle between the strands andthe wire fromgapproximately*145 to approximately 1,26?, `which `'decreases the stretch of the 'finishedbraid'up'to itsl'breaking pointA and JValso increases its strength. Although this type of vbraid gives poorcoverageand is too Vlooseior `ordinary insulation use, after'the-special-impregnation treatment' itl has la tensile strength offmore than 30 pounds and provides thetype'ofcovering which y is wire. l Y

The preferred tubular braid construction 'with needed-"toprotect the weakY resistance only eight'braidingendsis-rought on the outside l? and also on the inside. *For Vmany `applications it is desirable to provide Vva 'smooth 'outer surface. "This mayfbefaccmplished by providingthezcasing Li3 shownin Fig. 2 in the form-ofa'coating of mar-weight vinyl chloride resin with. a minimum thickness .of 10110 malres :a vlery vsatisfactory f coatingand this can Vreadily .be applied :as ,a

plastisol ortheextrusion process. Dielectrielacquer coatings may V'also :be applied Yin ,the usual manner. Y

' *Theroughness ontheinsideof thetubular braid ypresents animportant advantageand I, accordinglygpreferito ileave: this roughness f intact. The advantage-resultingithereirom can best be illus- An antiparts of zthewire Atouching .the fsheet. When gradually heated by an electric current this wirev failed to act as a fuse as expected but increased in temperature with increases in current until it was red hot, and the temperature was 500 F.

5 above the melting point. With adequate support on a smooth surface the oxide layer on the outside oi the wire acts like the skin on a sausage to contain the molten metal and prevent rupture. When the same experiment was repeated with the wire laid against a rough .fsurface, rupture v' and interruption of thecircuit'to'ok place below the expected temperature. The following is offered as a possible explanation: the Wire heats -.unevenly, points touching the support being much cooler than those which bridge between rhighsspots. These areas which are not as directly cooled by conduction reach higher than the average temperatures and, due to the positive .temperature coefficient of resistance, develop more heat than adjacentspots. When the local -temperature is close Jto thermeltingpoint the;wire

.is tremendously weakened, and, V.being :unsupported collapses -.witha Jrapid. local reduction .in cross sectional area. Ihisgreatlyincreases.the

v heatdissipationat .the .spot of incipientifailure --andcauses .thewire to. quickly. reach-.a tempera- K `vture. at jwhich. it agglommerates .-inthelowfspots .and .ceases to be a .'conductor, .whereupon .the o .wholesystem ,quieklylcoolsrdi 40 ture can of course readily be varied .within Wide limits, asifor lexample if the p-roperproportions "offtin, lead and bismuth'areusedan'alloy'melting lat only "205ml". can be obtained. Y o

'My improved conductor *.and v`resistance wire k i may be used for many different"purposesparticularly as a heatingelementfor'variousappliances "suchas the heating tape shown 'in`.,1*"igs.`3"5' inclusive, v'or the lheating blanketjshowniagrammati'cally in Figs. '6 and 7. o o In` using the conductor and "resistance'wire'ithe device shouldjbe assembled-so'that'any'applied 'tensile stresses are vcarried `by :the casing and cannotecause` stretching of' the wire itself. `Since vthe-wire is broughtoutsi'dethe casing 'onlyiat 'J0 of makingelectrical connections, it isjatthese two points only that special precautions needbe taken. A variety of constructions Vhave lbeen found suitable to ac'complishthis-purpose. `In c Figs. :'3-5 inclusive,l Ijhave shown aheatingtape v'or ribbonin the form of vvan *elongateds'tripfor ribbon SIA VVhaving flongitudinal 'corrugations I5 formedltherein interconnectedby the at webbed portions I6. f Y lr :The bodyiof-'the strip orfribbonis madefof a n :plastic .material having fgood dielectric properties. :For .this pur.pose,`-lI .prefer -plas'ticiz'ed polytfvinyl :chloride Embedded iin Y:the l'plastic "Ibo'dy Aof the strip or .ribbonfin ialignmentlwithithe ribs H thereof tare 1the;conductors :or lresistance wires 1.0 si l. Where the casing 'I2 .i'siimpregnated '-:with plasticized polyvinyl chloride it can be .locally fbonded or 1 heatesealed yto the body ioftheiribbon by the/application of dielectric heatateachfend `ofthe xstrip, after Ystrippingith'e :wires'bare for .75 a short distance to make :connections to the the two ends of a given length, for the purpose vimpregnated with a wax and, therefore, cannot be made to adhere to the body of the strip I4. One end of the ribbon or tape I4 is provided with a plastic button I1 and the other end vwith a `conventional plug I8 having contact prongs I9.

v.The conducting wires I I are connected together and soldered inside the plastic button II in the manner indicated at so as to provide a complete circuit. The plastic button II is secured by fusing to the plastic body of the ribbon as shown, so thattension forces applied to the button will be transmitted to the body of the ribbon. The latter is strong but has such a low modulus of elasticity vthat', it readily stretches when stressed. Thus a tension force applied to the button tends to slip the body of the ribbon along the casings I2 and would stretch or break the wires I I. To prevent this action the casing I2 is unraveled from the connected ends of the conductor as shown at 2I and is embedded in a suitable plastic or resin coating, which adheres thereto, such as polyamide. The polyamide resin is lapplied hot kand strikes through the wax impregnation, adhering to the glass fibers and serving as a stop to limit the outward movement of the plastic body of the ribbon.' Inthis manner any tensile loads will be transmitted through the glass braid casing and do not break or stretch the conducting wires.

Provision must also be made at the plug end of the "tape to transmit the tensile stresses through the glass braid casing rather than through the conducting wires. This may take the form shown in Figs. 3 and 4 in which the two outer resistance elements are extended beyond the plastic body of the tape and tied into simple knots as shown at 23 and a flexible acetate cement is preferably applied to the knotted area of the glass braid casing. lProvided the braid is not unraveled at the point the knot is tied,

r such knots do not adversely aiect the wire.

The glass casing preferably terminates just beyond the knotted area, but the conducting wire II is extended therebeyond and is folded back upon the resistance element in a small loop as shown. The looped portion of the conductor and the entire resistance element immediately beneath the knot is then encased in the tubular sleeve 24 of a solderless terminal connector 25 which, in turn, is connected by means of a screw 21 to the end of prong I9. The tubular neck 24 is tightly compressed around the resistance element to insure contact with the wire and prevent corrosion and so that if any attempt is made to withdraw it from the end of the element it will engage the knot 23. The neck portion of the terminal connector is preferably encased in an insulating sleeve 2'I made of a vinyl resin or the like.

The two adjacent or inner resistance elements of the tape are extended slightly beyond the body of the tape at the plug end thereof as shown in Fig. 3 and the glass braid casing is unraveled therefrom and the two wires are secured together as shown at 28. The connected wires and the unraveled glass casing are then embedded in polyamide resin which adheres to the glass fibers as shown at 29. The polyamide serves to protect the connected wires when theyare inserted in the plug. Also, due to the adherence of the polyamide to the glass fibers it serves as an additional stop to prevent outward movement of the plastic body of the tape.

If tensile forces are applied to the plug I8 they will be transmitted to the terminal connectors 25 which, in turn, will transmit them to the glass braid casing due to the interengagement of the necks of the terminal connectors with the knots 23. Thus, it will be seen that tensile stresses are not transferred to the conductors but are carried directly by the glass braid casing.

When my improved conductor and resistance element is used in a heating blanket such as shown diagrammatically in Figs. 6 and 7 provision must likewise be made so that tensile stresses cannot stretch the wire. For this application I prefer to use a 4% antimonial lead wire having a diameter of .0104" and a resistance of approximately 1.43 ohms per foot. With a two inch spacing between the paths of the element this resistance results in the desired heat dissipation of watts on 115 volts, if twoparallel circuits are used. The stress-carrying casing is preferably in the form of a braid With 11 pics to' the inch using glass ber yarn having a yield of about 7500 yards per pound and a tensile strength in excess of 20 pounds. This braid is impregnated with a vinyl emulsion and the resin is set by the application of heat, followed by cooling. These operations are completed before releasing the tension which is applied as the braid is Withdrawn from the braiding machine. Since the greatest possible flexibility is desired for the finished resistance element, but additional electrical insulation and mechanical protection are regained, I prefer to apply an outer casing of highly plasticized vinyl chloride in a thin layer, the resin being applied as a plastisol by dipping and curedfby heating under tension to 350 F., the element being cooled before the tension is released. The 'element I0 so formed is shown in Figs. 6 and 'I stitched to a suitable fabric 30.

The two terminal ends of the resistance element are arranged so as to be in proximate relationship as shown in Fig. 6 and are then connected to conventional lead wires 3l having an electric plug'32 at the end thereof.

The connection between the two ends of the resistance element I0 and the lead wires 3i is Y' shown'generally at 33 and is illustrated in detail in'r Fig; 7. This connection is similar to the connection between the resistance elements and the prongs of the plug I8. Thus, the resistance elements are tied into simple knots 23 and the knotted area is impregnated with a exible cement. The conducting wires I I are extended beyond the knotted areas and folded backwardly in a loop against the resistance element. The looped wire and resistance element immediately beneath the knot is encased in the sleeve 24 of the terminal connector 25 which, in turn, is connected to the lead wire 3l.

Thus any forces in tension applied to the lead wires 3| will be transmitted through terminal connector 25 to the glass braid casing.

The heating tape shown in Figs. 3 to 5 is somewhat similar to that shown in my copending patent application Serial No. 81,550 and is used in a similar manner. Thus, the plug I8 is connected to a suitable electric outlet and due to the fact that the conductors have been connected :blanket using .my neW conductor" 'andi resistance element, but .are :not :shown ,since they 1 form :no

part ofgmyinvention.

- '.'My improved Yresistance element 'provides 1a high degree rof. safety'against.firefhazardsinthat '..itWilLreadily fuse i' and :open 'the circuitA if iinadivertently connected .to 1to0. high 0apotential Vor yif yafslfiort circuit should.developdueito'any mechanvical :damage vto ithe Ninsulation for if .the device! is :allowedto operate under conditionsevvhichzrestrict "theclieat .dissipation `and Vgive rise tto excessive temperatures. Y

l .My .'improved conductor and resistance' Wire can .heuse'd formany otherjpurposesiparticularly `-Wliere Ait \is desired .to .have .aj-heatingrelement 4:which is .safe .in operationxand which "can withstand reasonable tensile stresses 'and loads. l. It "Will Ythus jbe. seen 'that I haveprcvided an'improved self-fusing electrical conductor: and resistance-Wire.' having a tension or :stress member' "iassociatedtherewith .in such a .manner .that fthe tensile .stresses are sustained directly thereby. .;Mo'dications Ymay be `made in ithe .illustrated y", embodiment.of my invention- Without departing -v from .theinvention as .set forth inthe accompany- :135

eingfclaims. f i,

Il.. 'Anrelectrical heating element comprising 'a lpluralityof elongated, attenuated. 'fragile'wires arranged inparallel relationship,;separate stress :carrying Vinsulating casings substantially :non- Jstretchable vin a longitudinal direction in the Storm '.of glass libre tubular braids .disposed 1 around the vaforesaid Wires with theglass nbre strands disposed'ata vrelatively small Vangle ofr Y less .than 45' to the longitudinal/axis ofthe braid,

'asecond-unitary casing made for material hav- -ing'a .lowermodulus `of Velasticity than .the glass .fibre 'tubular braids disposed around Vth'evirst .-'casings and Wires and :arranged in the'formof a corrugated elongated ribbon, a plug having Aelectrical contacts connected :to 'certain Vofthe wires at one end of the-Unit, .certain oithelwires :being connected together at 'the "opposite end to VVprovide atcompleteelectricalcircuitl and insulat-,

zing'. .means :encasing :the Wires atizthersaidopposite :tend :of said`V unit, saidzplug;andiinsulating` means ih'avingimechanicalconnection with the stress carrying: insulatingfcasingsiadj acent athe twdends of .theaunitavherebyztensile stressesapplieditoithe :two i ends fo'f zthe- 'unitnaretcarried vdirectly by the stress carrying casings.- 1" Y i 2. An'electrical "conductor comprising ian elonfgated'attenuated, :fragile lmetal vwire, aistre'ss carrying insulating -casing substantially vnon- Astretchable in a longitudinal direction in the form 'of a .tubular braidxiisposedaroundltheivvire Vand .made of glass '.'bre Vstrands disposed iat-a relatively; small'angleof less`than=45 tothe lon- -gitudinal` axis of the ibraid, Aa .second'insula'ting fand protective 'casing-'made of a'- Amaterial Vhavingfailower modulus of elasticity disposed outside the iirstcasing and* means :for Vtransferring "toltheiirst `casingany tensile stress which may 'be appliedto the second'casing.v f Y Y "3. vAnelectricalconductor asset-forth in claim 12in" Which knots are formed 4'adjacent the `two "ends-olf the y'rst casing in engagement-Withfthe seodn'd'casing vfor"transferring tensile stressfrom A'the 'firstA casing to thejsecond casing. Y

f' DUNCAN B. COX.

Name f :'.Date :396,473 'Gardner- 1 po,ct. ,14; 1884 V11,692,445 "Hope YNov/20, 192% 1,720,755 .fBehrensl ..-1 .Ju1y;16, .1929 1,957,565 I Wheeler May f8, 1934 l 2,057,689 Moss i oct. 20.1936 2,230,481 Bromley Feb. 4,"1941 40 [2,234,560 Keyes Mar. V11,V 1941 .12,243,560` A.Hall et al. May 27, '1941 21,250,024 s Hau etai. oct. 21, 1941 "2,390,039 .Slayter etal. r Nov. 27, 1945 2,397,568T v Seaman v Apr. Y2, 19.46 l 2,4U1'9,'655`- VReiser Apr. 29, 1947 4712,;4345937 register? Jan.. 20, 194s Y FQREIGN Peri-"Emis J'Number "Country #Date 435,557L @areas-Britain -"sept.f-23,"1935 Y* OTHER REFERENCES Owen-s,-(forning :Fiberglas Corp.; 

